The graphic card or graphic board provided with personal computers functions as a hardware interface with a display device. Most of the modem graphic boards have graphic accelerators which provides the ability to quickly move data to video memory from other places in the computer. Furthermore, the recent trends in display circuitry designs have focused on minimizing the need to move large blocks of video data from one place to another. By constructing and manipulating the on-screen image with processor power that is directly to video memory, massive transfers of video data can be minimized. Because less data moves across the system bus, there is less of a handicap from bus overhead no matter whether the I/O bus or a local bus is used as the transfer channel. A typical graphic board is shown in FIG. 1 for the purpose of explanation. The graphic board 101 comprises a graphic accelerator 100, a clock generator 30, a video memory 40, a RAMDAC 50, and a BIOS ROM 60. A local bus, for example the PCI bus to connected to the graphic accelerator 100 and the BIOS ROM 60 of the graphic board. The video data processed in the graphic accelerator 100 is suppled with a display monitor 11 through the RAMDAC 50 that includes a color look-up table and a digital-to-analog converter.
Especially, the graphic accelerator 100 has been improved to contribute to three dimensional games or graphics. In order to implement the 3-D graphics, well known texture mapping techniques are widely used. In brief, the texture mapping allows the graphic accelerator to calculate the memory address of the desired 3-D image to read out the texture data stored in the video memory 40 and again write into the frame buffer.
During the texture mapping, the performance of the graphic board has a great influence on how fast the graphic accelerator reads out the desired texture data or image from the video memory. Thus, it is desirable that the time required for accessing the video memory be reduced as much as possible.